Fuel cells are an attractive mechanism to generate electricity. One particular benefit of fuel cells over other energy sources is that they are more environment-friendly than batteries or combustion-based devices. Fuel cells have been successfully utilized that have relatively large sizes (e.g., dimensioned with a scale above centimeters). Fuel cells have a strong potential for use in smaller scales as well, because of their simple construction compared with internal combustion engines and the high energy density of their fuels compared with batteries. Unfortunately, miniaturization below the centimeter-scale has been unsuccessful mainly because one could not miniaturize the ancillary parts (e.g., pumps, valves, etc.) needed to operate the fuel cell and package them into a small space without taking up the volume needed for fuel.
Recent attempts have been made to pump fuels with no ancillary parts, opening the door for fuel cells in millimeter scales. For example, self-pumping fuel cells have developed that embed the ability to pump liquid fuel and remove generated gas bubbles using the anode side of the device. U.S. Patent Application Publication No. 2008-0118790 , for instance, discloses a device for pumping liquids using directional growth and elimination of bubbles. Without any discrete pump, the fuel can nevertheless be actively pumped and circulated, maintaining the fuel concentration. In contrast, passive fuel cells rely on fuel diffusion to the electrode, with the inevitable tendency to develop a depletion zone over time.
In order to eliminate the ancillary part for oxidant supply, attempts have been made to flow an oxygen-saturated oxidant inside the channel or integrate an air-breathing cathode to supply oxygen from ambient air. However, the mass-transport-rate-limited process to the cathodic site predominantly controlled the current density of those fuel cells. The fuel cells based on dissolved oxygen were particularly limited by the low solubility of oxygen. Air-breathing designs need a stream of electrolyte on the cathode that blocks fuel from crossing over and conducting ions through between the electrodes. These designs are also inherently dependent on the free convection of oxygen from ambient air to the cathode. Therefore, the system will lose its flexibility in choosing operating environments, and scaled-up applications will face difficulties, where most likely the fuel cells will need to be stacked on top of each other. The requirement to have ample air convection makes it very problematic to stack fuel cells into larger systems.
Thus, while progress has been made on the fuel side (i.e., anode) of a fuel cell for active pumping without the need for ancillary components, the oxidant side (i.e., cathode) still requires cumbersome construction and ancillary parts for the active supply of oxidants unless there is access to ambient air. Even if the system is designed to have access to ambient air, the performance may suffer if the oxidant supply is hindered (e.g., limited access to air due to flooding or the like). There is a need for a fuel cell that can be made with no moving parts and is able to scale-down to sub-centimeter dimensions.